This invention relates to optical systems and optical fibers, and particularly to optical fibers woven into other material to provide sensors or "smart" skins for aircraft and other applications.
Fiber optic sensor technology has become increasingly desirable for monitoring for numerous applications such as aircraft and spacecraft. The size, weight, communications density, immunity to interference, and ruggedness, are pushing fiber optic technology into more and more applications.
A recent concept in the manufacture of aircraft and spacecraft has been the employment of fiber optics within the skin of the craft itself, thereby creating a "smart" skin which enables sensors embedded into the composite material to convey information about the aircraft or spacecraft throughout the craft without need for separate communications links and their associated disadvantages.
The mechanical properties of cloth woven from glass fibers are reasonably well known. Such material provides desirable mechanical properties including high tensile strength, flexibility, resistance to weather as well as chemicals, high tear strength, dimensional stability, and abrasion resistance.
It is also known that individual optical fibers can be used to transmit optical signals throughout the length of the fiber and have very high bandwidths. Individual optical fibers have excellent optical properties, but are very fragile. A variety of techniques have been developed to hold individual fibers in a manner to prevent damage to them. For example, they are frequently encased in cables or other protective material. In addition, individual fibers can be grouped together to provide cables capable of carrying increased amounts of information.
One technique widely used for protection of optical fibers is to encapsulate them in an epoxy material to provide rigidity and strength. For example, U.S. Pat. No. 4,547,040 describes an optical fiber assembly where optical fibers are held in an embedding material.
Individual optical fibers have also been woven into sheets. For example, U.S. Pat. No. 4,907,132 describes a device where optical fibers are woven into a panel. The fibers are positioned in the warp direction of the weave. Where the fibers cross the woof fibers, the coating is removed so that the fibers emit light. In this manner, a panel made from the fibers emits light. U.S. Pat. No. 4,885,663 shows woven optical fibers where the bends in the fibers where they cross the woof provide discontinuities for the emission of light. The purpose of this structure is provide a light-emitting panel.
Other prior art such as U.S. Pat. Nos. 4,952,020 and 4,468,089 show optical fibers which are encapsulated in various ways to form cable assemblies such as described above. Unfortunately, cable assemblies such as described in these patents are relatively expensive and cannot be used to form sheet-like structures.
Many papers have been written on the application of optical fibers to the formation of "smart" skins for aircraft or spacecraft. In "Fiber Optic Skin and Structural Sensors," by Eric Udd, Industrial Metrology 1 (1990) 3-18, the use of optical fibers in a skin-like material for use as sensors is described. The paper, however, describes the fibers as being merely embedded in a structural material. Embedding the fibers in that manner suffers from the disadvantages discussed in the paper discussed below.
In a paper entitled, "Smart Skins and Fiber-optic Sensors Application and Issues," Kausar Talat, Boeing Defense & Space Group, Seattle, Wash. (unpublished), describes material with embedded optical fibers where the physical properties of the fiber itself were used as a sensor. The composite described in this article includes optical fibers disposed inside a laminated structure. At the end of the structure, the optical fibers pass through a tube inserted to prevent micro-bending of the fiber where it exists from between the laminated sheets. As described in the article, the laminated structure causes the fibers to kink during curing, creating losses as well as having other disadvantages discussed in the paper.